The influence of impurities on the physical properties of YBa2Cu3O7−δ superconducting ceramics is investigated. Samples of yttrium ceramics with different Ni, Zn, Co, Fe, and Ga impurity contents were prepared for this study, and the superconducting fraction, the low-frequency susceptibility in an external dc field, and the temperature dependence of the magnetic susceptibility in a weak ac field were measured for each sample. The last of these measurements permits a more precise determination of the critical temperature (the start of the transition) without regard for percolation effects. The experimental results show a stronger suppression of the superconducting state by nonmagnetic impurities, thus confirming the presence of d-state pairs in these superconductors. The degree of suppression of superconductivity by magnetic impurities depends on the preference of the impurities to locate in Cu(1) or Cu(2) sites.
Percolation processes and the magnetic susceptibility are investigated in composites based on yttrium high-Tc superconductors (Y-HTSCs). The superconducting fraction in the composites is determined for different amounts of an added nonsuperconducting fraction. The limit of percolation conduction via superconducting grains for a HTSC–insulator composite is (17±2)% superconducting fraction by volume. The Josephson properties of composite superconductors near the percolation limit are investigated.
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